<?php
/*
# 212. 单词搜索 II

# 题目
给定一个 m x n 二维字符网格 board 和一个单词（字符串）列表 words， 返回所有二维网格上的单词 。

单词必须按照字母顺序，通过 相邻的单元格 内的字母构成，其中“相邻”单元格是那些水平相邻或垂直相邻的单元格。同一个单元格内的字母在一个单词中不允许被重复使用。

https://leetcode.cn/problems/word-search-ii

提示：
- m == board.length
- n == board[i].length
- 1 <= m, n <= 12
- board[i][j] 是一个小写英文字母
- 1 <= words.length <= 3 * 10^4
- 1 <= words[i].length <= 10
- words[i] 由小写英文字母组成
- words 中的所有字符串互不相同


# 示例
```
输入：board = [
    ["o","a","a","n"],
    ["e","t","a","e"],
    ["i","h","k","r"],
    ["i","f","l","v"]
], words = ["oath","pea","eat","rain"]
输出：["eat","oath"]
```

```
输入：board = [
    ["a","b"],
    ["c","d"]
], words = ["abcb"]
输出：[]
```

# 解析



# 代码
```php
```